The invention relates to a method for producing homogenous material with an extruder at least two annular conical feed gaps situated between a rotatable rotor and a stator so that the material to be extruded is supplied between the rotor and the stator.
The invention further relates to an extruder comprising at least two annular conical feed gaps situated between a rotatable rotor and a stator, and which comprises means for supplying the material to be extruded between the rotor and the stator.
The invention also relates to a multilayer plastic pipe comprising an inner and outer pipe and an intermediate layer formed of softer material and situated between the inner and outer pipe.
U.S. Pat. No. 3,314,108 discloses an extruder comprising two conical stators and a conical rotor that is placed rotatably between the stators. The plastic material to be extruded is supplied via two separate conduits to different sides of the rotor. The rotor is provided with screw-shaped grooves by means of which the material is transferred towards an orifice at the narrower end of the extruder. The intermixing of the materials to be fed via different conduits is prevented with seals at the end where the material is supplied, wherefore the materials situated on different sides of the rotor only come together after the rotor near the end of the extruder, wherefore the intermixing of the materials remains insignificant. Further, unequal pressures may easily occur on different sides of the rotor due to the feed rates of the separate material flows fed from different conduits, and the pressure difference stresses the rotor and may damage it.
European patent application 89 906 779.7 discloses an extruder comprising several conical stators, and conical rotors that are rotatably placed between the stators. The material to be extruded is supplied along one conduit to the beginning of each rotor, whereupon the inlet of the rotor comprises openings via which the material can also flow to the other side of the rotor. By means of grooves provided on each side of the rotor, the material is transferred towards the end of the extruder. Also in this apparatus, the material flows passing on different sides of the rotor come together only near the orifice, wherefore the materials situated on different sides do not intermix efficiently. Further, there may be a great difference in pressure on different sides of the rotor and it stresses the rotor and may damage it.
When multilayer pipes are prepared, the intermediate layer or layers are typically made less strong than the outer and inner layers, or less strong materials are commonly used in the intermediate layer compared to the outer and inner layers. The material of the intermediate layer is preferably for example waste plastic, since this arrangement decreases the pollution of the environment and is also very advantageous economically. Another alternative is the foaming of the plastic material of the intermediate layer, but the foaming can naturally also be performed on waste plastic. However, especially the use of waste plastic requires that the equipment to be used has better mixing and processing properties than usually. Also, it may be necessary to mix reinforcing fibres in the intermediate layer, and the even distribution of the fibres in this layer requires very efficient mixing.
The extrusion apparatuses for multilayer pipes that have been developed so far have not provided sufficient mixing of the intermediate layers in the aforementioned cases.
U.S. Pat. No. 4,364,882 discloses a conventional foamed pipe made of PVC. The PVC is foamed to the density of 500 kg/m3, which in fact represents the minimum that can be achieved with the conventional technology. A typical foaming degree for PVC foam is 57%, in which case the density is 800 kg/m3, since with lower values the strength properties of the PVC foam deteriorate rapidly. The patent discloses a pipe having an outer diameter of 315 mm and the following structure:
the inner pipe has a thickness of 1.25 mm, the intermediate layer has a thickness of 9 mm and the outer pipe 1.25 mm. The total thickness of the pipe is thus 11.5 mm, the total weight is 7.63 kg/m and the obtained saving in weight compared to a conventional pipe having corresponding rigidity is 29%.
As the above example shows, foamed plastic provides considerable saving in material (i.e. in costs) while producing a lighter pipe structure. The use of such xe2x80x9cinferiorxe2x80x9d material as an intermediate layer is appropriate in this connection, since this layer is the least subjected to mechanical stresses, such as wearing and stress deformations, and to physical and chemical stresses, such as UV radiation and other impurities.
On the other hand, when the foaming degree of the intermediate layer is increased, i.e. the density is decreased, the properties of the foamed material also deteriorate significantly. Until now the maximum foaming degree used has been in practice such that it has decreased the density of the material into half of the density of unfoamed material, since after this level the strength of the foam deteriorates significantly, and the construction of a pipe with a strong structure has thus been considered impossible.
The above-described known multilayer pipes with a foamed intermediate layer have several disadvantageous properties, the most important of which is low impact resistance. The reasons for such poor mechanical strength include for example the poor homogenization of the plastic material.
The purpose of the present invention is to provide a method for producing homogenous material with an extruder, and an extruder in which the material supplied thereto can be mixed efficiently and which comprises none of the aforementioned problems.
Another purpose of the invention is to improve known multilayer pipes.
The method. according to the invention is characterized in that at least one rotor or stator comprises openings between the delivery point of the material to be extruded and the nozzle of the extruder in the direction of travel of the material, so that at least some of the material to be extruded can be made to flow through the openings.
Further, the extruder according to the invention is characterized in that at least one rotor or stator comprises openings between the delivery point of the material to be extruded and the nozzle of the extruder in the direction of travel of the material, at least some of the material to be extruded being able to flow through the openings.
Further, the multilayer pipe according to the invention is characterized in that at least one layer consists of fibre-containing solid or foamed reprocessed plastic.
The manufacture of the pipe utilizes the method and apparatus according to the invention.
The essential idea of the invention is that between the conical annular feed gaps there is a rotor or a stator comprising openings via which the material situated on different feed gaps can flow. Further, the idea of a preferred embodiment is that the rotor and the stators comprise oval notches that are positioned in such a way that they partly overlap so that in these notches the material supplied to the extruder can be mixed. The idea of another preferred embodiment is that several different agents are supplied to the extruder and they can be intermixed. The idea of a third preferred embodiment is that the material to be extruded is fed into the narrow end of the rotor, and the grooves of the rotor transport the material along the first side of the rotor to the wide end of the rotor where the material can enter the other side of the rotor via the openings provided in the rotor, whereafter the rotor grooves transport the material along the other side of the rotor and out via the nozzle of the exturder.
The invention has the advantage that the material to be supplied can be mixed efficiently, whereupon very homogenous material can be discharged from the extruder. A further advantage is that the openings provided in the rotor or stator between the feed gaps balance the pressures on different feed gaps, whereupon material can be supplied with different pressures to different feed gaps, for example. The advantage of another embodiment is that the oval notches provide even more efficient mixing of the material. By means of the invention, several different agents can be mixed and an efficiently blended homogenous mixture can be discharged from the extruder. By supplying the material to the narrow end. of the rotor, it is possible to provide a long mixing section, thus producing a very homogenous mixture. Further, it is possible to utilize the overheat generated during the final stage of the extrusion to melt the material supplied to the extruder. Another advantage is that the removal of gas and moisture from the material to be extruded can be arranged In the extruder at the point where the material is transferred via the openings in the rotor to the other side of the rotor.
Especially good mechanical properties of a multilayer plastic pipe are provided by the embodiment of the invention where the outer pipe and the inner pipe are reinforced by means of orientation and/or oriented fibres. Pipes that have been extruded in a conventional manner do not usually employ staple fibres for reinforcing the pipe mainly because during the extrusion the fibres are oriented according to the pipe axis and the reinforcing characteristic of the fibres is of no use. This concerns both pressure pipes and sewage pipes that require reinforcement in the circumferential direction. With certain techniques, such as rotating mandrels or rotating nozzles, the fibres can be oriented partly in the circumferential direction in the surface layers of the pipe by means of discontinuing the rotating motion. The use of this kind of defiberization in thermoplastic pipes for sewage is completely unknown due to the high costs of the process, for example. Fibre-containing masterbatch is typically about 3 to 4 times more expensive than conventional polyethylene. Adding fibres thus increases the price of the pipe, since the increased strength properties are not sufficient to compensate for the higher price of the raw material. According to the invention, it has surprisingly been found that the adding of fibres does produce a less. expensive final product than the conventional pipes. This is due to the fact that the reinforcing effect of the fibres is great in the layer that bears most of the load in the pipe according to the invention, i.e. in the inner and outer pipe, and for example in underground sewage and drainage pipes which are designed by using the ring rigidity as the criterion, the intermediate layer of the pipe is not used efficiently.